Nowadays, renewable resources are considered to be the new challenge in the development of Sustainable/Green chemistry. Interest in the application of biomass has increased considerably during the last decade because biomass-based resources are renewable and CO2 neutral. In addition, the projected long-term limitations on the exploitation of fossil feedstock, the recent increases in crude oil prices and environmental concerns regarding the local air pollution, the global warming problems caused by CO2, the biodegradability and biocompatibility of many petrochemical based products have also played a role in this respect. Today the world production of renewable biomass is about 200·109 t/a of which 130·106 t/a are fats and oils and only 7% of the total biomass production capacity are used for food, feed and non-food applications. These figures compared to the world capacity of extracted fossil fuels which is only 7·109 t/a show the huge potential of renewable biomass for energy, chemicals and material production. According to the Directive 2003/30/EC of the European Parliament and of the Council by 31 Dec. 2010 biofuels shall be 5.75% of the transportation fuels and according to the US roadmap for biomass technologies—2020 vision goals, biofuels will meet 10% of the fuels, and biomass-based chemicals 18% of the chemicals in the US market.
Vegetable oils and their derivatives are important feedstocks for the industry with a broad spectrum of applications such as in foodstuff chemistry, pharmacy, cosmetics, plastics, detergents, biolubricants and in the energy field with the production of biodiesel mainly by trans-esterification reactions with methanol or ethanol to obtain fatty acid methyl (FAME) or ethyl esters (FAEE).
Catalytic hydrogenation of renewable vegetable oils and their derivatives constitutes a major unit operation in the chemical industry. Selective hydrogenation processes of alkyl esters of vegetable oils to transform multiple unsaturated fatty esters into a single unsaturated ester without increasing the saturated part are of greatest interest in the fields of biolubricants and biodiesel. The first aim of these processes is to improve oxidative stability because the relative rates of oxidation are 98 for methyl linolenate (MLN), 41 for methyl linoleate (ML) and 1 for methyl oleate (MO) [Knothe, Fuel Process. Technol. 86, p1059f (2005); Frankel, Lipid Oxidation, The Oily Press, Dundee, Scotland, 1998]. Nohair et al. [J. Mol. Catal. A: Chem. 229, p117f (2005)] and Ucciani [Stud. Surf. Sci. Catal. 41, p26f (1988)] have reported that the oxygen absorption rate in linolenic acid (C18:3; 9c, 12c, 15c), linoleic acid (C18:2; 9c, 12c) and oleic acid (C18:1, 9c) is 800/100/1, respectively. The bis-allylic positions in common polyunsaturated FAMEs such as MLN (two bis-allylic positions at C-11 and C-14) and ML (one bis-allylic position at C-11) are even more prone to autoxidation than allylic positions. Therefore, partial hydrogenation of polyunsaturated FAMEs to C18:1 substantially increases their oxidation stabilities and greatly improve the ageing/storage properties of biolubricants and biodiesel which makes the addition of synthetic antioxidants superfluous. Falk et al. [Eur. J. Lipid Sci. Technol. 106, p837f (2004)] partially hydrogenated polyunsaturated FAMEs and could increase the oxidation stability at a low pour point of the biodiesel product. The second one aim is to avoid deterioration in low-temperature behaviour such as on the pour point. To preserve fluidity it is mandatory not to increase the melting point of the mixture that depends on both the amount of saturated methyl stearate (MS), C18:0, (melting point of MS=+39.1° C.) and the extend of cis/trans and positional isomerization (e.g. the melting point of MO, (C18:1, 9c), is −19.9° C. and of methyl elaidate (ME), (C18:1, 9t), is +10.0° C. The third aim of the partial hydrogenation of polyunsaturated FAMEs is to increase the performance i.e. the cetane numbers of biodiesel. Knothe reported for ethyl linolenate a cetane number of 22.7, for ML: 38.2, for MO: 59.3 and for methyl stearate a cetane number of 86.9.
In hydrogenation processes of C═C units in unsaturated fatty acids of vegetable oils are commonly used heterogeneous catalytic systems based on nickel, palladium, copper, copper-chromite, platinum etc. However, for edible oil hydrogenation heterogeneous catalysts based on nickel has been the choice of industry. The aims of traditional selective hydrogenation of edible oils are to increase their melting temperature and thus increasing the consistency for use as margarine and to improve the oxidative stability while an important amount of the C═C units in the fatty acid chain is cis/trans-isomerized. In recent years the negative health effects of trans-fats received increasing attention and are considered to be even more detrimental than saturated fats. Both trans- and saturated fatty acids contained in margarine are strongly correlated with a higher concentration of plasma LDL-cholesterol. The decisions in Europe to limit or in USA to declare the trans-isomers contained in fatty foodstuffs caused a demand for hardstocks with lower trans-isomers content. Therefore, there is increasing interest in the development of new industrial hydrogenation processes producing lower amounts of trans- and saturated fats. One development involves the use of selective homogeneous transition metal complexes as catalysts to obtain mainly cis-C18:1 fats and these homogeneous catalysts should be easily and quantitatively recovered and recycled. Ideal homogeneous catalysts for such conversions would be water soluble transition metal complexes to act in aqueous/organic two phase systems. The other development involves the use of shape-selective zeolites that allow the rather straight trans-isomer to enter the pores while keeping the more curved cis-isomer outside.
According to DE 4109246 A1 the hydrogenation of polyunsaturated fatty acids and their derivatives can be conducted using Na2PdCl4 catalysts precursors in propylene carbonate and aqueous sodium carbonate solutions; the catalytic reaction was performed in a homogeneous system. After the reaction, n-hexane was added to the reaction mixture and a two-phase system was formed, allowing the catalyst recovery by a phase separation of the lower propylene carbonate phase. Instead of propylene carbonate could also be used a nitrogenous aprotic solvent as activator such as dimethyl formamide. Fell et al. [Fat Sci. Technol. 92, p264f (1990); ibid. 93, p329f (1991)] also homogeneously hydrogenated ML using Ziegler-Sloan-Lapporte catalysts based on Ni(acac)2 or Pd(acac)2 and Al(C2H5)3 with a high selectivity (>90%) to C18:1 products. However, a shortcoming of this reaction is the cumbersome separation of the catalyst from reaction products and its quantitative recovery in active form as well as the large quantities of triethyl aluminium required i.e a molar ratio of Ni(acac)2/Al(C2H5)3= 1/10.
The complex object of the present invention has therefore been to provide a process for the production of unsaturated fatty acid alkyl esters having a total content of C18:1 of about 30 to about 80 Mol-% and more particularly more than 50 Mol-% by partial hydrogenation of unsaturated fatty acid esters having a total content of (C18:2+C18:3) of at least 65 Mol-% and more particular more than 75 Mol-% which avoids the disadvantages of the state of the art cited above. In particular, such process should allow to transfer highly unsaturated fatty acid alkyl esters or glycerides based e.g. on linseed or sunflower oil into a high-oleic fatty acid alkyl ester or glyceride, exhibiting low contents of higher unsaturated and fully saturated acyl moieties in order to improve the quality of said esters or glycerides, particularly with respect of their oxidative stability and cetane number at a low pour point for the application as so-called “biodiesel”, higher oxidative stability and low pour point for the application as “biolubricants” and improved oxidative stability and low contents of trans-fats and fully saturated compounds for the application of “edible oils hydrogenation”. Another object of the invention has been to conduct the hydrogenation process under mild and environmental-friendly conditions, particularly under biphasic conditions in an aqueous medium and by using catalysts which exhibit a high turnover frequency (TOF) and high selectivity towards the cis-C18:1 compounds and are easy to separate from the reaction mixture and to be returned into the process without reduction of activity.